In U.S. Pat. No. 4,213,590 assigned to the same assignee as the present application, a hitch head or fifth wheel is provided including a pair of pivotable jaws which hold in place a kingpin of a highway trailer. A horizontally movable lock block, having straight sides, biased by a stiff spring to closed position, engages the jaws, also having straight sides, and holds them in closed position, maintaining the kingpin in place. The lock block is movable to an open position by a finger extending through an opening in the lock block. The finger is rigidly attached to a horizontally extending operating shaft which is rotatable by a handle to move the finger to an overcenter position holding the lock block in open position. The finger includes a transversely extending protrusion which is engaged by the jaws as the kingpin exits to pivot the finger from the overcenter position and allow the lock block to move to a position engaging the rear portion of the jaws, holding the jaws open until another kingpin enters the jaws and pivots the jaws to closed position.
However, in this construction, because of the straight sides, the tolerances inherent in commercially manufacturing the jaws, lock block, spring and spring shaft sometimes cause an improper engagement or seating of the lock block upon the rear surfaces of the jaws, or excessive clearance between the lock block and jaws, allowing the jaws some free movement with the kingpin in place.
Furthermore, because of the straight sides, there is some tendency in the above construction for the rear portion of the jaws to provide a frictional drag on the lock block when the lock block is moved from the engaged position with the rear portion of the jaws to the disengaged position, and from the disengaged to the engaged position.
Also the straight sides do not provide a means to accommodate wear in the engaging portions of the lock block and the jaws.